A capacitive ceramic pressure sensor cell commonly comprises a ceramic substrate and a ceramic diaphragm which covers the substrate and is spaced from the latter to form a sensing chamber between the diaphragm and a surface of the substrate facing the diaphragm. The facing surfaces of the substrate and the diaphragm are provided with electrodes which together form a capacitor that provides an electric signal which corresponds to a pressure of a process medium acting on and deforming the diaphragm. Under overload conditions, the substrate serves as a limiter for the movement of the diaphragm.
To measure a difference between two pressures (differential pressure), two sensing chambers are commonly used, one for each pressure, the sensing chambers being spatially and mechanically connected with one another and being provided with one sensing capacitor each. In this manner it is possible to produce an electric signal which corresponds to the difference between a pressure acting on one of the sensing chambers and a pressure acting on the other sensing chamber.
A particular problem encountered with ceramic pressure sensor cells is to fasten and join the diaphragm in its edge area to the substrate in such a manner that the joint is gasu and liquid-tight and can withstand high tensile and compressive loads. In addition, the joint is to be long-term-stable and free of relaxation effects.
Glass-frit joints used in conventional ceramic pressure sensor cells do not fully meet the above requirements. Therefore, a joint produced by means of an active brazing solder has been used.
U.S. Pat. No. 5,050,034, for example, discloses a capacitive pressure sensor cell comprising
a ceramic substrate having PA1 a ceramic diaphragm PA1 a substrate having PA1 a first ceramic diaphragm PA1 a second ceramic diaphragm PA1 a ceramic substrate having PA1 a ceramic diaphragm having a planar inner surface, PA1 a ceramic substrate having PA1 a first ceramic diaphragm PA1 a second ceramic diaphragm PA1 a first ceramic substrate having PA1 a second ceramic substrate having PA1 a ceramic substrate which PA1 providing a ceramic substrate having PA1 depositing a first electrode on said concave central area; PA1 forming an electrical connection from said first electrode through said substrate to said second major surface; PA1 providing a ceramic diaphragm having a planar inner surface; PA1 depositing a second electrode on the central portion of said planar inner surface of said diaphragm such that, when said diaphragm is placed on said substrate, said second electrode extends up to said planar ring surface of said substrate; PA1 applying an active brazing solder to said convex portion of said substrate between said cylindrical surface and said planar ring surface; PA1 placing said diaphragm on said planar ring surface of said substrate such that said second electrode of said diaphragm extends up to said planar ring surface, and said second electrode faces said first electrode; PA1 heating said substraic and said diaphragm in a vacuum or inert-gas atmosphere until the active brazing solder has melted; and PA1 allowing said substrate and said diaphragm to cool down. PA1 providing a ceramic substrate, at the first major surface thereof, with a concave first central area PA1 depositing a first electrode on the first central area and providing a electrical connection from the first electrode through the substrate to the cylindrical surface of the substrate; PA1 providing the substrate, at a second major surface opposite the first major surface, with a concave second central area PA1 depositing a second electrode on the second central area and providing an electrical connection from the second electrode through the substrate to the cylindrical surface of the substrate; PA1 providing a first ceramic diaphragm congruent with the first major surface of the substrate, on a planar inner surface thereof, with a third electrode dimensioned so PA1 providing a second ceramic diaphragm congruent with the second major surface of the substrate, on a planar inner surface thereof, with a fourth electrode dimensioned so PA1 applying respective quantities of active brazing solder sufficient to braze the first and second diaphragms to the substrate to portions of the convex first surface of the substrate located between the first ring surface and the cylindrical surface and to portions of the convex second surface of the substrate located between the second ring surface and the cylindrical surface; PA1 placing the surface of the first diaphragm provided with the third electrode on the first ring surface of the substrate; PA1 placing the surface of the second diaphragm provided with the fourth electrode on the second ring surface of the substrate; and PA1 heating the substrate and the diaphragm in a vacuum or inert-gas atmosphere until the active brazing solder has melted, and then allowing them to cool down. PA1 providing a first ceramic substrate, at a first major surface thereof, with a concave first central area PA1 depositing a first electrode on the first central area and providing an electrical connection from the first electrode through the first substrate to a second major surface of the substrate opposite the first major surface; PA1 providing a second ceramic substrate, at a first major surface thereof, with a concave second central area PA1 depositing a second electrode on the second central area and providing an electrical connection from the second electrode through the second substrate to a second major surface of the second substrate opposite the first major surface; PA1 providing a ceramic diaphragm congruent with the first major surface of the first substrate, on a planar first surface thereof, with a third electrode dimensioned so PA1 providing a planar second surface of the diaphragm opposite the first surface with a fourth electrode dimensioned so PA1 applying respective quantities of active brazing solder sufficient to braze the first and second diaphragms to the substrate to portions of the convex first surface of the substrate located between the first ring surface and the cylindrical surface and to portions of the convex second surface of the substrate located between the second ring surface and the cylindrical surface; PA1 placing the first surface of the diaphragm, provided with the third electrode, on the first ring surface of the substrate; PA1 placing the second surface of the diaphragm, provided with the fourth electrode, on the second ring surface of the substrate; and PA1 heating the substrate and diaphragm in a vacuum or inert-gas atmosphere until the active brazing solder has melted, and then allowing them to cool down.
a cylindrical surface and, PA2 at a first major surface, a central area which PA2 which is joined to the substrate using a plane-parallel ring of active brazing solder to form a high-vacuum-tight sensing chamber, PA2 an edge area and, PA2 at a first major surface, a concave first central area which PA2 said substrate further having, at a second major surface opposite the first major surface, a concave second central area PA2 the substrate being provided with a connecting channel between the first central area and the second central area; PA2 which rests on and is fixed to the first ring surface of the substrate, PA2 which rests on and is fixed to the second ring surface of the substrate, PA2 a cylindrical surface, PA2 a first major surface and PA2 a second major surface, PA2 a first electrode located in said concave central area of said first major surface, and PA2 an electrical connection extending from said first electrode through said substrate to said second major surface; and PA2 a second electrode located on said planar inner surface of said diaphragm, PA2 said planar inner surface of said diaphragm resting on said planar ring surface of said first major surface of said substrate, PA2 said diaphragm being joined to said substrate by an active brazing solder PA2 a high-vacuum-tight sensing chamber being formed between said planar inner surface of said diaphragm and said first major surface of said substrate, and PA2 electrical connection to said second electrode being made through said circumferential wedge zone. PA2 a cylindrical surface and, PA2 at a first major surface, a concave first central area which PA2 which substrate further has, at a second major surface opposite the first major surface, a concave second central area which PA2 said substrate further having a connecting channel between the first central area and the second central area; PA2 which rests on the first ring surface of the substrate, and PA2 which is joined to the substrate on the first ring surface and between the cylindrical surface and the first ring surface by means of active brazing solder forming a first circumferential wedge zone, to form a first high-vacuum-tight sensing chamber, PA2 with a third electrode being provided on a planar inner surface of the first diaphragm facing the substrate, PA2 which rests on the second ring surface of the substrate, and PA2 which is joined to the substrate on the second ring surface and between the cylindrical surface and the second ring surface by means of active brazing solder forming a second circumferential wedge zone, to form a second high-vacuum-tight sensing chamber, PA2 with a fourth electrode being provided on a planar inner surface of the second diaphragm facing the substrate, PA2 a first cylindrical surface and, PA2 at a first major surface, a concave first central area which PA2 second cylindrical surface and, PA2 at a first major surface, a concave second central area which PA2 rests with a first surface on the first ring surface of the first substrate, PA2 is joined to the first substrate on the first ring surface and between the first cylindrical surface and the first ring surface of the first substrate by means of active brazing solder forming a first circumferential wedge zone, to form a first high-vacuum-tight sensing chamber, PA2 rests with a second surface on the second ring surface of the second substrate, PA2 is joined to the second substrate on the second ring surface and between the second cylindrical surface and the second ring surface of the second substrate by means of active brazing solder forming a second circumferential wedge zone, to form a second high-vacuum-tight sensing chamber, PA2 the first surface of the diaphragm being provided with a third electrode PA2 the second surface being provided with a fourth electrode PA2 a cylindrical surface, PA2 a first major surface and PA2 a second major surface, PA2 which, in the direction of and up to a cylindrical surface, merges into a convex first surface having a first vertex line, PA2 which, in the direction of and up to a cylindrical surface of the substrate, merges into a convex second surface having a second vertex line, PA2 that, after the first diaphragm has been placed on the first ring surface of the substrate, the third electrode extends up to said first ring surface; PA2 that, after the second diaphragm has been placed on the second ring surface of the substrate, said fourth electrode extends up to said second ring surface; PA2 which, in the direction of and up to a first cylindrical surface, merges into a convex first surface having a first vertex line, said convex first surface being formed as a first planar ring surface in the area of the first vertex line; PA2 which, in the direction of and up to the second cylindrical surface of the second substrate, merges into a convex second surface having a second vertex line, PA2 that, after the diaphragm has been placed on the first ring surface of the first substrate, said third electrode extends up to said first ring surface; PA2 that, after the diaphragm has been placed on the second ring surface of the second substrate, said fourth electrode extends up to said second ring surface;
is provided with a first electrode and PA3 has an electrical connection from the first electrode through the substrate to a second major surface, and PA3 with a second electrode being provided on a planar inner surface of the diaphragm facing the substrate. PA3 is provided with a first electrode, PA3 has a first electrical connection to the first electrode, and, PA3 in the direction of the edge area, merges into a convex first surface PA4 which has a first vertex line intersecting the edge area and PA4 forms a first planar ring surface in the area of the first vertex line, PA3 which is provided with a second electrode, PA3 has a second electrical connection to the second electrode, and, PA3 in the direction of the edge area, merges into a convex second surface which PA4 has a second vertex line intersecting the edge area and PA4 forms a second planar ring surface in the area of the second vertex line, PA3 with a third electrode being provided on a planar inner surface of the first diaphragm facing the substrate; and PA3 with a fourth electrode being provided on a planar inner surface of the second diaphragm facing the substrate. PA3 said second major surface being opposite said first major surface, PA3 said first major surface including a concave central area which, in the direction of and up to said cylindrical surface, merges into a convex surface having a vertex line, PA3 said convex surface forming a planar ring surface in the area of said vertex line, PA3 which forms a circumferential wedge zone between said diaphragm and said substrate in the area of said substrate between said planar ring surface and said cylindrical surface, PA3 is provided with a first electrode, PA3 has a first electrical connection from the first electrode through the substrate to a second major surface, and, PA3 in the direction of and up to the cylindrical surface, merges into a convex first surface having a first vertex line, PA4 said convex first surface forming a first planar ring surface in the area of the first vertex line, PA3 is provided with a second electrode, PA3 has a second electrical connection from the second electrode through the substrate to the cylindrical surface, and, PA3 in the direction of and up to the cylindrical surface, merges into a convex second surface having a second vertex line, PA4 said convex second surface forming a second planar ring surface in the area of the second vertex line, PA3 to which third electrode contact is made through the first wedge zone; and PA3 to which fourth electrode contact is made through the second wedge zone. PA3 is provided with a first electrode, PA3 has an electrical connection from the first electrode through the first ceramic substrate to a second major surface opposite the first major surface, and, PA3 in the direction of and up to the cylindrical surface, merges into a convex first surface having a first vertex line, PA4 said convex first surface forming a first planar ring surface in the area of the first vertex line; PA3 is provided with a second electrode, PA3 has a second electrical connection from the second electrode through the second ceramic substrate to a second major surface opposite the first major surface, and, PA3 in the direction of and up to the second cylindrical surface, merges into a convex second surface having a second vertex line, PA4 said convex second surface forming a second planar ring surface in the area of the second vertex line; and PA3 to which contact is made through the first wedge zone, and PA3 to which contact is made through the second wedge zone. PA3 said second major surface being opposite said first major surface, PA3 said first major surface including a concave central area which, in the direction of and up to said cylindrical surface, merges into a convex surface having a vertex line, PA3 said convex surface being formed into a planar ring surface in the area of said vertex line; PA3 said convex first surface being formed as a first planar ring surface in the area of the first vertex line; PA3 said convex second surface being formed as a second planar ring surface in the area of the second vertex line; PA3 said convex second surface being formed as a second planar ring surface in the area of the second vertex line;
The joint produced by means of active brazing solder meets the above-mentioned requirements for high stability, but in certain cases where the diaphragm is subjected to overpressure, it has turned out that the diaphragm cannot be supported on the substrate in a satisfactory manner. Because of the "angular" shape of the ring of active brazing solder, which serves as a spacer between the substrate and the diaphragm, tensile stresses may occur, particularly in the edge region of the diaphragm, which result in a failure of the diaphragm.
U.S. Pat. No. 4,329,826 discloses a capacitive differential pressure sensor cell comprising:
In the case of this prior-art differential pressure sensor cell, the ring surfaces, which serve exclusively to join the respective diaphragms to the substrate, extend up to the cylindrical surface of the substrate. The way the joint is produced is not explained.
It has turned out that the joint produced solely by means of the ring surfaces is insufficient, particularly if great axially parallel forces act on these surfaces. In addition, such a joint is not high-vacuum-tight and not long-term-stable.